Semiconductor integrated circuits are typically manufactured using photolithography techniques. In a photolithography process, a photoresist layer is deposited on a substrate, such as a silicon wafer. The substrate is baked to remove solvent remained in the photoresist layer. The photoresist is exposed through a photomask with a desired pattern to a source of actinic radiation. The radiation exposure causes a chemical reaction in the exposed areas of the photoresist and creates a latent image corresponding to the mask pattern in the photoresist layer. The photoresist is next developed in a developer solution to remove either the exposed portions of the photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The patterned photoresist can then be used as a mask for subsequent fabrication processes on the substrate, such as deposition, etching, or ion implantation processes.
Immersion lithography is a photolithography resolution enhancement technique that the semiconductor industry has embraced for the 45 nm and 32 nm nodes, and possibly beyond. In an immersion lithography process, a liquid medium is placed between the final lens of the lithography tool and the wafer, replacing the air medium in a traditional dry lithography process. Compared to a dry lithography process, the immersion lithography can increase the resolution by a factor equal to the refractive index of the liquid. Current 193 nm immersion lithography tools use water as the liquid medium which has a refractive index of 1.44 at 193 nm.
One of the main challenges to the adoption of the 193 nm immersion lithography is defect control. Water has been shown to extract photoacid generators (PAGs) and photo generated acids from photoresist. The leaching of the PAGs and the acids into the water can create defects in the photoresist. In addition, the extracted PAGs and acids can also contaminate or corrode lens of the lithography tool. To battle against these concerns, a topcoat layer has been used to place directly on top of the photoresist. The topcoat serves as a barrier between the water and the photoresist and can effectively reduce the leaching of the PAGs and the acids from the photoresist into the water. The topcoat is generally an acidic material which can be removed by the aqueous base developer during the develop step of the photoresist.
To increase the feature density in an integrated circuit, the semiconductor industry has developed various double patterning techniques such as double-exposure and double-exposure double-etch. In a double-exposure process, a photoresist layer is exposed by a sequence of two separate exposures using two photomasks. This technique is commonly used to produce patterns in the same layer which are different or have incompatible densities or pitches. In a double-exposure double-etch process, a first layer of photoresist is exposed. The pattern formed in the first photoresist is transferred to an underlying hardmask layer by a first etch process to form a first pattern in the hardmask. A second layer of photoresist is coated on the hardmask. The second photoresist undergoes a second exposure to form a second pattern in the second photoresist. The first pattern in the hardmask and the second pattern in the second photoresist form a combined mask which is transferred to the final layer underneath in a second etch process. The double-exposure double-etch technique allows an increase in feature density.
Although double patterning techniques work relatively well in enhancing pattern density, there is a general concern that the CD of the pattern formed after the first exposure may vary during the subsequent processes, resulting in an additional source of CD variation.